In a radar instrument for detecting a target object using a high frequency signal, a slot antenna or the like is typically provided to an end of a waveguide, for example. The high frequency signal is transmitted out of the radar instrument via a slot of the antenna. For such a radar instrument, in order to improve the flexibility of attachment of the instrument, a coaxial line is typically used for transmission of the high frequency signal through the radar instrument. In this case, conversion of the high frequency signal is necessary between the waveguide and the coaxial line and, thus, a coaxial waveguide converter is used therebetween. The coaxial waveguide converter typically carries out impedance matching between the waveguide and the coaxial line to optimize the transmission efficiency.
FIG. 9 shows a vertical cross-section of a conventional coaxial waveguide converter 100. The coaxial waveguide converter 100 typically includes a waveguide 10, an outer conductor 20, an inner conductor 30, and a matching section 13. Hereinafter, the configuration of this conventional coaxial waveguide converter 100 will be described in detail.
An insertion hole 11 is formed in the waveguide 10, and a coaxial line including the outer conductor 20 and the inner conductor 30 is attached to the waveguide 10 through the insertion hole 11. The inner conductor 30 is provided coaxially with the outer conductor 20 and is fitted inside the outer conductor 20 via an insulator 12. An end of the inner conductor 30 projects from the corresponding end of the outer conductor 20 into the waveguide 10 via the insertion hole 11. Although the coaxial line is connected to the waveguide 10 via a known coaxial connector, the description and illustration of the coaxial connector are omitted herein. In addition, the matching section 13 for adjusting impedance is provided to the waveguide 10. A load component (not illustrated), such as a slot antenna, is typically attached to the waveguide 10 via the matching section 13.
In the case of such a conventional coaxial waveguide converter 100, a redundant space where the matching section 13 is placed must be provided. In addition, according to the load (e.g., slot antenna) and the frequency of the high frequency signal which passes through the coaxial waveguide converter 100, the matching section 13 may be necessary to be changed in its size and shape. Therefore, various types of the matching sections 13 may be manufactured and prepared in advance to meet the requirements immediately.
To get around this requirements, JP 2007-88797(A) discloses a coaxial waveguide converter which can adjust the impedance corresponding to the various loads and frequencies without providing the matching section 13.
The coaxial waveguide converter disclosed in JP 2007-88797(A) has a short board provided inside a waveguide, which is movable in the axial direction of the waveguide. In addition, the position of a probe provided to a tip end of an inner conductor is adjustable in a direction perpendicular to the axial direction of the waveguide. That is, the distance between the probe and the short board, and the projected length of the probe which projects inside the waveguide can be adjusted arbitrarily. Therefore, the impedance matching according to the load and frequency can be achieved with a single type of the coaxial waveguide converter.
However, even for the coaxial waveguide converter disclosed in JP 2007-88797(A), because the short board must be provided, the redundant space must still be provided. In addition, prior to changing the projected amount of the probe, the coaxial waveguide converter must be disassembled. Particularly, if one just wants to tune the probe finely, the disassembly is very troublesome. Further, when the coaxial waveguide converter is used with a radar instrument having a rotary antenna section, the position of the short board and/or the position of the probe may shift due to the rotation or vibration caused by the antenna section. Therefore, the impedance matching may be difficult to be carried out.